An oceanic core complex preserved in the Squanga Lake ophiolite, northern Atlin terrane, Yukon

Abstract

The Squanga Lake ophiolite of southern Yukon exhibits a non-Penrose pseudostratigraphy with mantle tectonites separated from upper crustal rocks by an extensional detachment zone (Squanga Lake detachment zone, SLDZ), compatible with formation in an oceanic core complex. Spinel, orthopyroxene and clinopyroxene mineral compositions indicate dynamic re-crystallisation during exhumation of ophiolitic mantle rocks. The rocks and structures of the SLDZ also preserve evidence of exhumation: 1) clasts of primitive olivine-cumulates in the SLDZ mélange represent excised lower oceanic crust; 2) structures in the SLDZ indicate ductile to brittle conditions; 3) temperatures along the SLDZ span from upper- to lower-amphibolite facies conditions. We obtained 248.77 ± 0.22, 248.53 ± 0.41, and 248.84 ± 2.09 Ma U-Pb zircon ages from upper crustal gabbro, a gabbro fragment within the SLDZ mélange and a dyke within the SLDZ implying syn-kinematic magmatism. Calc-silicate alteration associated with cross-cutting diabase dykes yielded a UPb titanite age of 247 ± 10.5 Ma. All igneous rocks show similar geochemical signatures with ubiquitous subduction input. Our data indicate that Squanga Lake ophiolite preserves an oceanic core complex, formed in a back-arc basin environment.